Newtons third law pulleys

1. Oct 17, 2007

emma3001

Blocks X and Y of masses mx=5.12kg and my=3.22kg are connected by a fishing line passing over a frictionless pulley. Show that block X slides up the incline (35.7 degrees above the horizontal) with positive acceleration. Determine the magnitude of the acceleration. (0.273m/s2 is the answer)

i want to find the gravitational force for mass x so Fg=5.12x9.8=50.2N
For mass y f=mg =3.22x9.8=31.6N

Just like with projectile problems you need to find the x and y components of Ftension for block x,which is getting pulled up at an angle but how do i do that if i dont know the applied force?

Last edited: Oct 17, 2007
2. Oct 17, 2007

learningphysics

So the block y is hanging vertically?

Write the $$\Sigma F=ma$$ equation for the block y, and the $$\Sigma F=ma$$ equations for block x

Call the tension T. The acceleration of block y downward equals the acceleration of block x up the plane. Call this a.

Is the incline frictionless?

You have 2 unknowns T and a which you should be able to solve for with the equations you get.

3. Oct 17, 2007

emma3001

yes, the incline is frictionless and block y is hanging vertically. if i know that the normal force of y is 31.6N, does that help me in any way?

4. Oct 17, 2007

learningphysics

If block y is hanging... how is there a normal force?

The way you described the problem... there are 2 forces acting on y... the weight, and tension... write the $$\Sigma f = ma$$ equation for y.

5. Oct 17, 2007

emma3001

oops... i guess i meant the gravitational force is 31.6N.

6. Oct 17, 2007

emma3001

if i only have the weight of block y how am i able to find out FT? After all, isnt FT calculated by saying

Fnet=FT-Fg

do i not have 2 variables here?

7. Oct 17, 2007

learningphysics

yes... I'll call FT, T...

Fnet = T - my*g

now... I'm going to take a as the downward acceleration... taking up as positive and down as negative:

my*(-a) = T - my*g (which has two variables, a and T)

can you come up with an equation for block x?

Last edited: Oct 17, 2007